Many suspense and science fiction novels have characters locked in small, sealed rooms, waiting for rescue or looking for a way out. Find out how long a person would realistically have in a situation like this, and why oxygen deprivation is only half the problem.

So you've got your character stuffed in an airlock. This kind of thing always happens in sci-fi. While you're pondering whether to blow them out into the pitiless emptiness of space or have them saved by a comely alien lass or commanding space captain, you need to figure out how long you can keep them there. Most of us spend our days indoors, but we open those doors - or let air pass through the cracks - enough that we don't have to consider our oxygen. Someone in a sealed room does, and deeply. Every time they breathe in, they're taking in some oxygen and breathing out less.

At first, it doesn't seem that bad. The atmosphere is about 20 percent oxygen. People breathe this in, and breathe 15 percent oxygen out, making the air that's left lower in Good Ol' Oh Two, but still quite breathable. Each minute a person at rest takes in roughly seven to eight liters of air, which adds up to about 11,000 liters of air a day. That sounds like a lot, volume-wise it's only 388 cubic feet. A ten by ten by ten foot room has 1000 cubic feet of air. Add that to the fact that you'll be breathing out a lot of oxygen, and you only need about 19 cubic feet of pure oxygen a day. Your breathing may get labored by the end of the second day, but a relatively small room should be fine for about three days. right?

It should be, but biology is cruel. People don't just breathe in oxygen. They breathe out carbon dioxide. In the atmosphere, carbon dioxide makes up about 0.05 percent of the gas composition. The exhaled breath of a human being has roughly five percent carbon dioxide. That's a hundredfold increase. Carbon dioxide is a dangerous poison to the body. Low level exposure causes muscle twitching, high blood pressure, and reduced brain activity. Higher levels reduce the ability of the body to regulate the heartbeat, cause convulsions, and will eventually kill a person. Once the carbon dioxide levels in the room rise to two percent, carbon dioxide poisoning starts to happen. That occurs when the overall oxygen level falls to 19 percent, about half way through the first day.